1. Technical Field
This invention relates to the art of resistance welding or to brazing of metals and more particularly to the integration of welding and spot brazing of light metals, particularly aluminum or aluminum alloys, which are to be used in structural, energy absorbing applications.
2. Discussion of the Prior Art
Resistance spot welding is the primary fastening method in the automobile industry for structural members. The characteristics of such joining method is well documented in welding handbooks and is well suited to joining high resistance metals such as uncoated, low carbon steel. Difficulties arise when applying such spot welding to low resistance metals, such as aluminum. First, resistance heating, needed to effect the weld must be provided by the electrodes, which heating affects electrode tip life as well as weld quality. Secondly, strength of an aluminum spot weld in shear is related to weld nugget diameter. The aluminum spot weld will be composed of a soft cast metal that extends substantially through most of the thickness of the workpiece; increasing the strength of the spot weld will require increasing the diameter of such weld nugget. This is usually not practical because the weld nugget size is set by material and joint geometry, as well as by spot welding schedules. Thus spot welding of aluminum-type workpieces may result in poor strength welds.
Resistance brazing joins work pieces by melting only an interposing filler metal using an electric current passing through the joint itself. Heat is generated primarily by high resistance of the electrodes rather than at the interfaces of the workpieces. Equipment is the same as that used for resistance welding and the pressure needed for establishing electrical contact is ordinarily applied through electrodes; electrode pressure provides a tight joint fit needed for capillary movement of the melted filler metal in the joint. Resistance brazing is desirable because it provides for more accurate control of heat input without heat influencing much of the workpieces, it is much faster in creating the joint, uses less energy than welding and can be easily controlled such as by robots. But spot brazing creates a small or thin fusion zone that is relatively low in strength.
Resistance brazing has been applied commercially to aluminum alloys only on a limited basis. The aerospace industry has utilized resistance brazing only under highly controlled conditions requiring a vacuum environment to insure the absence of oxides. Vacuum brazing is undesirable in the automotive industry because of its high cost, slowness in application, and the need for unusually strict cleanliness. In all other commercial applications, a flux is necessary to ensure dissolution of the oxides inherent on aluminum and to ensure wetting. Flux brazing is undesirable because of the need to remove flux residues to avoid corrosion problems, the presence of porosity in inclusions in the resulting joint, and the likelihood for a weaker joint.
Any attempts to avoid the presence of a flux or the use of a vacuum environment, have required the utilization of copper based filler metals. The presence of copper in the filler metal is undesirable because of electrochemical effects. Copper remains in some form, contiguous with aluminum or titanium, promoting corrosion in an automotive application because of galvanic corrosion. Similarly, the presence of magnesium in the filler metal, when used with aluminum workpieces, must be avoided or severely limited because it prohibits fluidity of the molten filler flowing into narrow gaps to bond or fuse readily.